In recent years, it has become common to mount a large package having more than 2000 terminal pins on a board (mainly motherboard) of a large-scale computing machine, such as a large-sized computer, because of an increase in the number of signals, and the size of such a large package has also increased to the extent of exceeding 40 mm×40 mm. Since the package is so large in size that it is equipped with a stiffener intended to prevent the warpage of the package as a whole.
The stiffener, after being solder-bonded to the motherboard, is attached to the motherboard with mechanical components for fastening purposes (mainly screws).
FIG. 3 is a schematic cross-sectional view illustrating a conventional package mounted module.
A package mounted module 20 illustrated in FIG. 3 has a structure wherein a package board 11 is solder-bonded to a motherboard 21 by the fusion and fixation of solder balls 15 on the underside surface of the package board 11, so that the entirety of a package board module 10 is mounted on the topside surface of the motherboard 21.
Note here that the package board module 10 has a structure wherein an LSI chip 12 is mounted on the topside surface of the package board 11 by the fusion and fixation of solder balls 13. In addition, the package board module 10 is provided with a stiffener 14 fixed in contact with the topside surface of the package board 11 so as to surround the LSI chip 12. The stiffener 14 is intended to support the package board 11 and prevent the warpage of the package board 11.
Furthermore, in the package mounted module 20 illustrated in FIG. 3, a stiffener 22 is disposed on the underside surface of the motherboard 21 in a position to interpose the motherboard 21 between the stiffer 22 and the package board 11.
The stiffener 22 is fixed to the motherboard 21 with fastening components (screws 23 here) to support the motherboard 21, thereby preventing the warpage of the motherboard 21.
FIG. 4 is a schematic view illustrating a positional relationship among respective members when the package mounted module 20 is viewed from above.
The stiffener 22 has an area wider than that of the package board 11, and the motherboard 21 and the stiffener 22 are fixed to each other with fastening components (screws 23 here) at multiple locations on the periphery of the stiffener 22 falling outside the package board 11 when a stack of the package board 11, motherboard 21 and stiffener 22 is viewed from above.
As described above, the stiffener 22 is fixed such that the periphery thereof falling outside the package board 11 is fastened with fastening components (mainly screws). Accordingly, stress arises in solder-bonded portions between the package board 11 and the motherboard 21 due to a temperature difference between a solder melting point and a normal temperature or a temperature difference between the points in time of apparatus operation and apparatus shutdown and due to external mechanical stress (connector plugging/unplugging, vibration, drop, or the like), thus significantly affecting the reliability of solder bonding.
This is especially true in recent years, as the use of lead has been prohibited by regulations on hazardous substances and high melting point solder such as tin-silver-copper solder (melting point: 218° C.) has come into use in place of conventionally used eutectic tin-lead solder (melting point: 183° C.). As a result, the range of temperatures from a solder melting point to a normal temperature has widened, thereby causing more intense stress to occur.
Note here that Patent Document 1 discloses a package structure wherein a first pressurizing force for joining a heat sink and a package board is separated from a second pressurizing force for pressurizing the package board to a printed board. In addition, Patent Document 2 discloses that, for the TAB tape side face of a stiffener, at least portions of the stiffener positioned immediately above solder balls are removed and dimples or through-holes are formed, thereby providing elasticity between the solder balls and the stiffener.
The technique disclosed in Patent Document 1 is intended to prevent the degradation of reliability due to pressurizing forces during bonding and is not intended to prevent the occurrence of stress after solder bonding.
The technique disclosed in Patent Document 2 is useful in preventing the occurrence of stress during bonding, but the structure disclosed therein is vulnerable to vibration since elasticity is provided between the solder balls and the stiffener.
Patent Document 1: Japanese Patent Laid-Open No. 2004-165586
Patent Document 2: Japanese Patent Laid-Open No. 11-220055